The survey of 2,000 users found that over 80 per cent no longer open attachments from unknown sources, and nearly half have stopped visiting sites they suspect of harbouring malware.
Nearly half of respondents claimed to have personal experience of a spyware infestation.
"This is probably a conservative estimate," said Susannah Fox, associate director at Pew.
"There is a very strong likelihood that a big proportion of those who have had computer problems have been victimised by spyware without knowing the cause of their problems."
Fox cited a survey last year from the National Cyber Security Alliance which found that 53 per cent of respondents said that they had spyware on their computers. But a remote scan of the PCs revealed that 80 per cent of respondents had such programs installed.
Over half of respondents had suffered slow downs or crashes more frequently as a result of spyware, and nearly one in five was having web pages redirected.
Eighty per cent of internet users (and 90 per cent of internet users who have been hit by adware) said that more should be done to alert consumers about adware in files they are downloading.
Only about one in 10 internet users indicated that the current practice of clicking through a user agreement or disclaimer is adequate consent to install adware on a computer.
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